1. Field of the Invention
The present invention relates to a transfer case (power distribution device) for four-wheel-drive vehicles.
2. Description of the Prior Art
As commonly used, there exists two types of transfer cases which are attached to the rear of the speed change gears, one with a center differential gear and the other without a differential gear.
An example of the conventional type is shown in FIG. 1. In this type, the power from an input shaft 101 is transmitted and distributed to the front propeller shaft 105 and the rear propeller shaft 107 via a transmission route 103. It is so arranged that the high speed range and the low speed range can be accomplished by coupling a first sleeve 109, which is installed within the transmission route 103, to the high speed gear 113 and the low speed gear 111, respectively, that are located on both sides of the sleeve 109. Furthermore, drive on the rear wheels alone can be realized by moving toward right to decouple a second sleeve 115 which is disengageable from the front propeller shaft 105.
In this type of transfer case 117, even if either of the front or rear wheels find themselves in a slipping situation, when the front wheel side and the rear wheel side are in the directly connected state while the vehicle is in the four-wheel-driving condition, there will be secured a sufficiently strong tractability even on a low-friction road surface, thanks to the distribution of the driving power in proportion to the gripping capability of the tires. On the other hand, during turn of the vehicle, there will arise a difference in rotations between the front wheels which require larger turn radius and the rear wheels which can do with smaller turn radius. This difference in rotations shows itself up as the torsional stress in the transmission route 103 producing torque. The torsional stress affects the driving system, for example, deteriorating the driving capability substantially. In particular, in the case of turning at a low speed, such as occurs in garaging a vehicle, the input torque succumbs to the torsional stress so that the vehicle will be brought to a stop under a circumstance similar to the one in which brakes are applied to the driving system. In addition, there remains undesirable features such as the overloading of the driving system, abnormal friction on tires, and an excessive force required for steering. In such a case, these problems will be resolved by switching to the two-wheel-drive. However, switching in the midst of garaging a vehicle or the like situation is troublesome, and also there will be required a fairly large power for such a switching.
On the other hand, in the transfer case 119 of the conventional type which can eliminate the rotation difference that occurs in such a case as turning, the power from the input shaft 101 is transmitted, as shown in FIG. 2, for example, to the front propeller shaft 105 and the rear propeller shaft 107, via a transmission route 103. Here, however, there is required a center differential gear 121 (a differential device) midway in the transmission route 103, in order to allow for the difference in rotations of the front propeller shaft 105 and the rear propeller shaft 107. The center differential gear 121 comprises a small differential gear shaft 123 which is incorporated into the input shaft 101, a small differential gear 125 which is mounted on the shaft 123, and a large differential gear 127 which engages with the small gear 125 and is incorporated into the rear propeller shaft 107. Thus, for example, by moving a sleeve 129, which is arranged in the prior stage of the input shaft 101, toward the right, it becomes possible to engage the sleeve with the spline of an output gear 131. This allows the transmission of the power, as shown by the thick arrowed line in the figure, to the output gear 131, the sleeve 129, and the input shaft 101 via an intermediate gear 135 which is engaged with an input gear 133, making the low speed range available for the system. At the same time, the power is transmitted, through the center differential 123, to the front propeller shaft 105 and the rear propeller shaft 107.
While the center differential 123 possesses an advantage of permitting to absorb the rotation difference which arises in turning the vehicle, it reveals a weakness in that, when the front wheels spin in a marsh or the like, the power from the input shaft 101 is consumed mainly for spinning the front wheels at a high speed, with little transmission of power to the rear wheels, which necessitates an inclusion of a differential locking mechanism into the system. Consequently, it will lead to a high cost and large size of the system which is undesirable in view of the contemporary trend for miniaturization and light weight for the products.